From Sahih Bukhari (Hadith 5261):
Narrated `Aisha:A man divorced his wife thrice (by expressing his decision to divorce her thrice), then she married another man who also divorced her. The Prophet (ﷺ) was asked if she could legally marry the first husband (or not). The Prophet (ﷺ) replied, "No, she cannot marry the first husband unless the second husband consummates his marriage with her, just as the first husband had done
Reference: Bukhari 5261